Such circuit boards are widely applied in high-frequency technology, especially in a large number of electronic devices used in high-frequency technology. The terminology, high-frequency, refers here to frequencies in the range of 3 MHz to 300 GHz. High-frequency components are circuits or circuit parts, which have at least one circuit component working at these frequencies.
An example of such devices are fill level measuring devices working with microwaves according to the pulse radar method. These serve for measuring fill level of a fill substance located in a container. They have arranged above the fill substance an antenna, via which in measurement operation, periodically, short microwave pulses of a predetermined transmission frequency are sent toward the fill substance, and their signal components reflected on the surface of the fill substance received after a travel time dependent on the fill level. These fill level measuring devices have regularly a high-frequency module, with which the transmission signals are produced and their reflected signal components conditioned and processed for travel-time measurement. The high-frequency module includes, for this, a number of high-frequency components, such as, for example, a transmitting circuit for producing the transmission signal, a receiving circuit for receiving reflected signal components and processing circuits for determining travel time.
High-frequency components, as a rule, emit high-frequency electromagnetic interference waves during operation. Conversely, however, exactly high-frequency components are frequently very sensitive to high-frequency interference.
Accordingly, it is exactly in high-frequency technology especially important to suppress interfering effects of high-frequency components arranged on one and the same circuit board on one another.
In such case, it is to be noted that high-frequency electromagnetic interference fields are, as a rule, capable of propagation both above the circuit board as well as also within individual plies of circuit boards.
Various solutions are known from the state of the art for reducing mutual degrading of high-frequency components arranged on one and the same circuit board.
Thus, for reducing mutual influencing of neighboring components, an as great as possible spatial separation between the components can be provided. This is, however, only possible when sufficient space is available on the circuit board.
Furthermore, there can be inserted between neighboring components a comparatively broad copper conductive trace on a circuit board ply, in order to effect a blocking of wave fractions propagating above the circuit board ply. For blocking wave fractions propagating within the circuit board ply, there can be provided along the copper conductive trace vias, which connect the copper conductive trace conductively with a copper ply located under the circuit board ply. In such case, a section of the circuit board ply located between two vias acts as a metal hollow conductor, whose cut-off frequency is adjustable by its dimensioning and the dielectric properties of the circuit board ply. Correspondingly, interference signals with frequencies lying under the cut-off frequency are not capable of propagation in the hollow conductor.
Likewise it is known to shield the components from one another by solid metal webs applied on the circuit board or by soldered on, sheet metal pieces. Solid metal webs effect a high-quality interference suppression. They have, however, the disadvantage that they are costly and must in the case of each change of circuit board layout be taken into consideration, respectively, in most cases, also changed in position.
Moreover, described in German Patent, DE 10 2010 061 714 A1 is the placement on a portion of the circuit board of an at least sectionally metallized covering, which shields the therebeneath located portion from electromagnetic interference fields, which the circuit board upper side is exposed to, and suppresses an emission of electromagnetic interferences.
Additionally, the interior of the covering can be filled with a conductive foam, in which multiple reflections of electromagnetic waves take place, which lead to an undirected attenuation of the interference waves.
In such case, however, a minimum distance of a fourth-wavelength must be maintained between the foam and the neighboring high-frequency lines.
Also, these covers are costly and the geometry of the covering must be taken into consideration in the case of each change of circuit board layout, respectively, in most cases, the geometry must also be changed.